1. Field of the Invention
The present invention relates to a method of manufaturing a grain-oriented silicon steel sheet having excellent magnetic characteristics and capable of being used as a core material for a transformer or the like.
2. Description of the Related Art
A grain-oriented silicon steel sheet usable as a core material for various types of transformers possesses crystal grains highly integrated in an orientation which has an easily magnetized axis (110) [001] in the rolling direction, i.e., in a so-called Goss-orientation. Flux density reflects the degree of orientation in a steel sheet and is generally evaluated by a value B.sub.8 (T), showing the flux density in a magnetic field of 800 A/m.
A phenomenon known as secondary recrystallization is utilized to align crystal grains in the Goss-orientation. Secondary recrystallization involves an abnormal grain growth behavior which has a very strong orientation selectivity, wherein ordinary crystal grains (which are called primary recrystallized grains) are thermally grown. It is very important to control orientation selectivity and abnormal grain growth when seeking excellent secondary recrystallized grains having a high degree of integration in the Goss-orientation. For this purpose, it is important to maintain a delicate balance between aggregate structure, crystal grain size, and the restraining force of an inhibitor (the ability of an inhibitor to restrain precipitates as a dispersed second phase and the movement of a grain boundary due to the segregation of a component in the grain boundary). Proper balancing restrains the growth of crystal grains and the like in primary recrystallization prior to secondary recrystallization.
Although aggregate structure, crystal grain size and inhibitor restraining force may be adjusted by controlling hot rolling, cold rolling and primary recrystallization annealing such adjustments require fine control of temperature rolling reduction, and surface state control, and create problems in industrial scale production.
Since defective stripe-shaped secondary recrystallized crystals often grow along the rolling direction, secondary recrystallized crystals defectively grow over the entire sheet surface and the crystal orientation of secondary recrystallized grains varies greatly from the Goss orientation. As a result, magnetic characteristics deteriorate and a large amount of scrap iron is generated. Further, such fine control of parameters affecting magnetic characteristics in industrial scale manufacturing processes is often difficult or impossible to achieve, thereby creating problems in yield and quality control.
Japanese Patent Application Laid-Open No. 2-267223 discloses a means for controlling the conditions of primary recrystallization annealing so that primary recrystallized grains are controlled within parameters. The method involves the monitoring of grain size of the primary recrystallized grains through an on-line system. Further, Japanese Patent Application Laid-Open No. 4-337029 discloses a means for controlling primary recrystallization annealing temperature so that the primary recrystallized grain size is within a range of 15-25 .mu.m. The method involves measuring the N content of a steel sheet prior to final cold-rolling.
These prior art technologies focus on the conditions of primary recrystallization annealing (temperature and line speed) to stabilize and improve the magnetic characteristics of a product. The primary recrystallized grain size is controlled because it greatly affects the behavior of secondary recrystallized grains as described above. However, when the inhibitor restraining force is changed by variations in hot-rolling conditions and annealing conditions (including cooling conditions) after cold rolling, the optimal primary recrystallized grain size also changes in accordance with these variations. Therefore, magnetic characteristics cannot be stabilized even when defective growth of secondary recrystallized crystals can be restrained, thereby severely inhibiting the practical applicability of these technologies.
Other technologies for obtaining secondary recrystallized grains having excellent magnetic characteristics by controlling primary recrystallized grain size are known. For example, Japanese Patent Application Laid-Open No. 2-182866 discloses a means for controlling a grain size of crystals after primary recrystallization annealing to 15 .mu.m or more with a coefficient of Variation of 0.6 or less. Japanese Patent Application Laid-Open No. 6-33141 discloses a means for controlling average grain size after primary recrystallization annealing to 6-11 .mu.m with a coefficient of variation of 0.5 or less, which also involves increasing the average grain size 5-30% just before the start of secondary recrystallization. Japanese Patent Application Laid-Open No. 5-156361 discloses a means for controlling primary recrystallized grain size to 10-35 .mu.m before the start of final finishing annealing after primary recrystallization annealing. Japanese Patent Application Laid-Open No. 5-295438 discloses a means for controlling primary recrystallized grain size to 18-35 .mu.m.
Although these technologies seek to produce good secondary recrystallized crystals for improved magnetic characteristics by controlling primary recrystallized grain size, none has addressed the problem of unstable magnetic characteristics arising in industrial scale production.
The present invention advantageously addresses these problems by balancing primary recrystallized grain size with the inhibitor restraining force to control secondary recrystallized crystals for the improvement and stabilization of magnetic characteristics.